1. Field of the Invention
The present invention relates to a brake system in a vehicle in which any one or both of a hydraulic braking and a regenerative braking can be selected based on the operation of a brake operating element and the travelling condition of the vehicle.
2. Description of the Prior Art
There are conventionally known vehicles which travel by an electric motor using a battery as an energy source and in which the battery is charged by an electric power generated by the motor by regenerative braking of driving wheel, thereby prolonging the possible travel distance of the vehicle for a single battery charge (for example, see Japanese Patent Publication No.6204/81 and U.S. Pat. No. 3,621,929).
It is conceived that in order to increase, to the maximum, the efficient of recovery of energy by the regenerative braking in such electric vehicle, the braking is usually performed in a regenerative braking-preference mode in which a regenerative braking force for the driving wheel exceeds a theoretical braking force distribution characteristic, and as required, the braking is performed in a usual mode in which a braking force distribution characteristic thereof corresponds to the theoretical braking force distribution characteristic. However, if the construction is adapted such that the regenerative braking-preferential mode and the usual mode can be switched over, a problem is encountered that the magnitude of the braking force is suddenly varied upon switch over of the modes from one to another and as a result, the smooth braking is hindered.
There is a conventionally known brake system in a vehicle comprising driving wheel capable of being hydraulically and regenerative braked and follower wheel capable of being hydraulically braked, in which regenerative braking of the driving wheel and the hydraulic braking of the follower wheel are performed simultaneously and in parallel during an initial braking, and when the regenerative braking force reaches a limit value, the hydraulic braking of driving wheel is started. In this brake system, however, a portion of the kinetic energy of the vehicle is consumed by the hydraulic braking of the follower wheel from the initial braking stage having the largest kinetic energy and for this reason, an effect of recovery of the energy by the regenerative braking cannot be exhibited sufficiently and hence, it is impossible to largely prolong the possible travel distance per one charging of the battery.
In an electric vehicle utilizing hydraulic braking and regenerative braking in combination, it is desirable, from the viewpoint to increase the efficiency of recovery of the energy, that the regenerative braking is used as much as possible, on the one hand, and it is desirable that the hydraulic braking of a high responsiveness and a large braking force is used during a hard braking and when there is a fail in the regenerative braking system, on the other hand. Particularly, when the regenerative braking is preferentially selected in order to increase the efficiency of recovery of energy, it is necessary to insure a sufficient braking capability on the supposition of an unlikely event when the regenerative braking system is failed.